The present invention relates to a method and a device for controlling a carburetor of an internal combustion (i. c.) engine in order to automatically adjust the mixture or air/fuel ratio thereof to a preferred level at different operational conditions.
In all i. c. engines the so-called air/fuel ratio is of great importance to the function of the engine. The air/fuel ratio is usually designated A/F. To obtain a proper combination of low fuel consumption, low exhaust gas emissions, good operability, and high power the A/F ratio must be kept within relatively close limits; cf. FIG. 3. As a rule, an A/F ratio slightly to the lean side of the optimal power value is preferred.
Future legal restrictions on emissions of CO will not allow manual adjustment of the carburetor. With the tolerances of manufacture of the carburetor that can be obtained it is not possible by using fixed nozzles in the carburetor to both fulfil the mentioned legal restrictions and simultaneously assure the operator a good operability at all combinations of air pressure, temperature, varying fuel quality, etc. The preferred A/F ratio is namely influenced by a number of factors. Some of these are known when the engine is designed and can therefore be corrected from the beginning, but others depend on variations of external circumstances such as air pressure, temperature, and fuel quality, and variations connected with the manufacture of the carburetor.
Theoretically it would perhaps be possible to calibrate each carburetor and use sensors for pressure, temperature, fuel quality, etc. but in practice this would be very expensive and the system would be complicated and have a low reliability of operation. As a consequence, certain i. c. engines are provided with special oxygen sensors or lambda sonds in the exhaust system. It is thereby possible to sense the performance of the combustion and the result of the measurements of the sond can be used in a control system to control the A/F ratio in order to provide a good result. The result of the oxygen sensor (lambda sond) is fed back to the fuel control system, whereby no further sensors are required. However, this is an expensive and complicated control system which for reasons of cost and operational reliability can hardly be used in such consumer products as motor saws, lawn mowers, etc.
The object of the invention is to reduce the above-mentioned problems essentially by providing a method and a device for controlling a carburetor of an i. c. engine by which the A/F ratio is automatically adjusted to a preferred level at different operational conditions without using an oxygen sensor (lambda sond). This object is obtained by a method and a device which according to the invention are characterized by the features set forth in the accompanying claims.